1. Field of the Invention
The present invention relates to a method for the recovery of dialkyl naphthalene-2,6-dicarboxylates from naphthalene-2,6-dicarboxylic acid-containing polyesters by the alcoholysis of the polyesters. In one aspect, the invention pertains to the recycling of dialkyl naphthalene-2,6-dicarboxylates from polyester scrap which may be contaminated with other polyesters, polymers, or dyes. In another aspect, the invention relates to novel naphthalene dicarboxylate compounds.
2. Description of the Background
Poly(ethylene terephthalate) (PET) polyesters are well known as packaging materials. However, gaseous diffusion rates across barriers of PET are higher than desired for materials to be used in packaging carbonated beverages. It has become established that polyesters made from naphthalene-2,6-dicarboxylic acid (NDA) show superior properties to those made from the much more commonly available, and much less costly, terephthalic acid. In particular, the polyester formed from NDA and ethylene glycol, poly(ethylene naphthalene-2,6-dicarboxylate) (PEN), is superior to PET in the retention of gaseous molecules in bottles fabricated from these polyesters. That is, gaseous diffusion rates across barriers of PEN are less than the rates for similar configurations of PET. Packages fabricated from PEN are expected to be more desirable, particularly for carbonated beverages. However, PEN has a higher value and cost of manufacture than PET. It is thus desirable, from an economic standpoint, to be able to recover and recycle bottles made from PEN. It is also desirable, from an environmental viewpoint, to be able to recover and recycle bottles made from PEN. However, although a number of methods have been devised for the recovery and recycling of PET, there is no known method for the recovery and recycling of PEN.
Typically, PET is recycled by converting the polyester to its precursors, dimethyl terephthalate (DMT) and ethylene glycol (EG) by reacting the polymer with methanol in the presence of a transesterification catalyst. On cooling the reaction mixture, DMT precipitates and may be collected by filtration. The crude alcohol-wet DMT filter cake is then washed with alcohol. The washing step removes most of the catalyst, glycol, and side products, and the crude DMT cake is then distilled to remove alcohol and finally vacuum distilled to give pure DMT. This recovery process is complex and time consuming, and therefore greatly reduces the incentive for the recycling of PET. If the washing step is omitted, the transesterification catalyst present in the DMT filter cake catalyzes the production of polyester during the recovery process. Thus, omission of the washing step decreases the yield of DMT.
U.S. Pat. No. 3,488,298 discloses an alternative method wherey the DMT may be distilled directly from the reaction mixture without first filtering and washing. The reaction mixture is separated into its components by distillation, with the catalyzed production of polyester during the distillation is prevented by first adding a catalyst poison. The disclosed catalyst poisons include a number of phosphorus-containing compounds.
U.K. patent application No. 2,041,916 describes a method for improving the yield of DMT from the methanolysis of PET. The improvement comprises adding an alkaline transesterification catalyst to the filtrate obtained in the filtration step to catalyze the formation of DMT from the by-products of the initial methanolysis.
East German Pat. No. 116,251 describes the degradation of PET to polyesters which may be used in high quality fiber forming processes by reacting scrap PET with small quantities of alcohol at temperatures from 200.degree. to 300.degree. C. in an extrusion device.
Japanese Pat. Nos. 48 68,538 and 48 68,537 disclose a method for improving the yield of DMT by adding compounds such as ammonium hydroxide or alkali metal hydroxide or chloride to the cooled reaction mixture before filtration. Japanese Pat. No. 49 41,330 describes the alcoholysis of PET with ethylene glycol, and Japanese Pat. No. 50 82,028 teaches the alcoholysis of PET with a mixture of ethylene glycol and methanol in the presence of a transesterification catalyst.
Thus, the known methods for recovering and recycling PET either suffer from being impractical or involve complicated multi-step procedures. Further, it is not clear that any of the above-described methods would be applicable to the recovery and recycling of poly(alkylene naphthalene-2,6-dicarboxylate) resins. Thus, there is a need for a method for the recovery and recycling of poly(alkylene naphthalene-2,6-dicarboxylate) resins. In particular, there is a need for a method for the recovery and recycling of PEN.
Since it is possible that the PEN to be recycled will often be mixed with PET and other polymers and may be contaminated with impurities such as dyes, a method for the recovery and recycling of PEN from such mixtures is desired. Further, since the recycled PEN may be used for food packaging, it is desired that the recycling and recovery method involve conversion of PEN to a readily purifiable precursor so that contamination may be avoided.